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Offline Mr Andrew

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What is energy?
« on: 26/12/2007 14:00:28 »
What is it?  We say, "this has more energy than that," and "mass is energy" but what are we talking about?  Energy is the ability to do work?  What is work...a change in energy!


 

Offline lightarrow

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What is energy?
« Reply #1 on: 26/12/2007 14:53:31 »
What is it?  We say, "this has more energy than that," and "mass is energy" but what are we talking about?  Energy is the ability to do work?  What is work...a change in energy!
Work is F*s; F = force, s = displacement.
 

lyner

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What is energy?
« Reply #2 on: 26/12/2007 16:02:59 »
One all-embracing definition is "what is needed to make anything 'happen'".
 

Offline Soul Surfer

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What is energy?
« Reply #3 on: 29/12/2007 12:32:54 »
For all particles the energy of the particle depends on three things: firstly its mass, this is called the mass energy;  secondly its velocity with respect to other significant objects in its vicinity,  this is its kinetic energy; finally any energy associated with its position with respect to gravitational (or electrical and magnetic fields if it is charged and/or possesses magnetic field) fields in the vicinity.  This is called its potential energy. all of these can be increased or decreased by interactions with other partcles or objects in the vicinity but the sum total of energy within any closed system will always remain the same.
« Last Edit: 29/12/2007 12:35:56 by Soul Surfer »
 

another_someone

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What is energy?
« Reply #4 on: 29/12/2007 13:37:13 »
One all-embracing definition is "what is needed to make anything 'happen'".

This is what I would view as the best definition - and ofcourse, the converse of it is energy is that which has caused something to happen.

From this, one can philosophically argue for a conservation of energy to be a necessity of science, since a violation of a conservation of energy leads to an asymmetry of causation (i.e. if that which required to cause something to happen is inconsistent, then one cannot create a systematic mathematical model of the universe; and similarly, if that which is necessary to cause something to happen is in some way other than the consequence of the converse of it happening, then you again are unable to create a mathematical model of the universe).
 

Offline lightarrow

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What is energy?
« Reply #5 on: 29/12/2007 16:47:14 »
One all-embracing definition is "what is needed to make anything 'happen'".

This is what I would view as the best definition - and ofcourse, the converse of it is energy is that which has caused something to happen.
I would view it as the worse definition. Which is the exact meaning of the term 'happen'?
 

another_someone

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What is energy?
« Reply #6 on: 29/12/2007 17:19:42 »
One all-embracing definition is "what is needed to make anything 'happen'".

This is what I would view as the best definition - and ofcourse, the converse of it is energy is that which has caused something to happen.
I would view it as the worse definition. Which is the exact meaning of the term 'happen'?

OK - I did not choose the specific word, but essentially I understand the overall phrase to mean "that which is required to achieve a particular change".
 

Offline McQueen

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What is energy?
« Reply #7 on: 30/12/2007 13:28:34 »
The conclusion drawn by Einstein in his study of special relativity was that a body gaining kinetic energy increases its mass. Thus heating a body increases its mass a little. Radiation emitted by the sun contains energy and by strength of this has mass. The term for the relation between mass and total energy is Einstein's mass energy formula:
E = mc^^2
It follows that the total energy of a body is proportional to its mass. The massess of all bodies increase with the increase in their energies. In general any variation in the energy of a system(body, particle etc.,) in any form entails a proportional variation in mass.
 

Offline lightarrow

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What is energy?
« Reply #8 on: 30/12/2007 20:59:07 »
The conclusion drawn by Einstein in his study of special relativity was that a body gaining kinetic energy increases its mass. Thus heating a body increases its mass a little. Radiation emitted by the sun contains energy and by strength of this has mass. The term for the relation between mass and total energy is Einstein's mass energy formula:
E = mc^^2
It follows that the total energy of a body is proportional to its mass. The massess of all bodies increase with the increase in their energies. In general any variation in the energy of a system(body, particle etc.,) in any form entails a proportional variation in mass.

I don't enjoy having always to correct you, but I have to do it, in cases like this one, sorry!

1. A body gaining kinetic energy do really increases its mass *but only* if the body's center of mass remains stationary (so it can only have rotational kinetic energy).
The same for any kind of body (even massless): it increases mass, when it acquires energy, only if that body's total momentum is and remain zero:

E2 = (cp)2 + (mc2)2

2. Heating a body do increases its mass but not its kinetic energy.

3. Electromagnetic radiation going in a specific direction don't have mass; but if you consider a system of two beams of light going in two different directions, then this system do really have mass (it's weird, I know).

4. The total energy of a body *is not* proportional to its mass, in general; it is only if the body remains stationary. The formula E = mc2 should be canceled from the books (especially those for medium and high school students) and replaced with the one I've written up. Infact E = mc2 is just the particular case of
E2 = (cp)2 + (mc2)2 when momentum p is = 0.
 

Offline Mr Andrew

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What is energy?
« Reply #9 on: 02/01/2008 02:32:04 »
Energy is the ability to do work.  Therefore, if mass inherently has energy, this explains how the field it generates does work on other mass.  By the same token, charge has energy because it generates a field which does work on other charges.  Thoughts?
 

Offline Soul Surfer

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What is energy?
« Reply #10 on: 02/01/2008 09:42:39 »
We live in a dynamic universe and the kinetic energy of the particles and waves within it is its most fundamental feature. Mass is a secondary feature that (somewhat unexpectedly) was shown to have an equivalence to energy by relativity theory.

The simplest experiment that puts charge in context is that of pair production.  If a high enegy gamma ray interacts with another charged particle or atom it is possible for the gamma ray to lose some of its enegy this to be converted into an electron positron pair and nothing else.  The energy budget of this interaction Which has been measured many times does not include any energy for the production of the pair of opposite charges only the changes in momentum and energy and the mass of the interacting particles.

In higher energy interactions it is also possible to produce pairs of more massive particles in exactly the same way.  However charges are aloways produced in exactly matching equal and opposite quantities by this type of interaction.

It follows that charge production does not have an energy budget asoociated with it unless you somehow include it in the mass energy of the particles but this is different for different particles.
 

another_someone

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What is energy?
« Reply #11 on: 02/01/2008 10:28:13 »
Charge itself is merely a force, and so has no energy or mass equivalence.

On the other hand, charge over distance (just as gravity over distance) does have an energy value, so would it not also alter the mass of the total system (i.e. two particles bound by charge having a different collective mass than the the sum of the two particles when separated)?
 

Offline lightarrow

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What is energy?
« Reply #12 on: 02/01/2008 17:01:32 »
Charge itself is merely a force, and so has no energy or mass equivalence.

On the other hand, charge over distance (just as gravity over distance) does have an energy value, so would it not also alter the mass of the total system (i.e. two particles bound by charge having a different collective mass than the the sum of the two particles when separated)?
Yes, this mass is in the electric field generated from the charges (very unintuitive concept, at least for me).
 

lyner

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What is energy?
« Reply #13 on: 03/01/2008 23:14:38 »
Quote
I would view it as the worse definition. Which is the exact meaning of the term 'happen'?
What I mean is whenever there is any process, change or occurrence (described by anything happening) energy is transferred. 'happen' is just a catch all word which includes everything. It is almost the opposite of the limited mechanical implication of 'work' which is what I was taught at School.
 

lyner

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What is energy?
« Reply #14 on: 03/01/2008 23:25:59 »
Charge itself is merely a force, and so has no energy or mass equivalence.

On the other hand, charge over distance (just as gravity over distance) does have an energy value, so would it not also alter the mass of the total system (i.e. two particles bound by charge having a different collective mass than the the sum of the two particles when separated)?
Charge is charge and force is force. Don't confuse the two. Dimensionally, they are different quantities.
Force is what you get when you put a charge in a field - Coulomb's law. Energy (work) is what is transferred when you move a force through a distance.
You really need to get the basics right when stretching classical Science to include 'modern' ideas and keep on the straight and narrow - else it gets like the less reputable posts we sometimes read on these pages.

The mean KE of a hot, 'stationary' object will be greater than the same object when cold - the internal particles will be vibrating etc more. This reflects the relativistic increase in mass, if you like.
 

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What is energy?
« Reply #14 on: 03/01/2008 23:25:59 »

 

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